One way of reducing the running noise of gear wheels is to use gear wheels with a helical toothing. In order to eliminate axial forces caused by this, the use of gear wheels with two helical toothings comprising single helical toothings which point counter to each other, in particular the use of gear wheels with a double helical toothing, is known. The double helical toothing is formed by two helical toothings, arranged alongside each other and pointing counter to each other, having identical reference circles.
In order to manufacture a gear wheel with a double helical toothing, U.S. Pat. No. 5,659,955, for example, discloses pressing and sintering circular cylindrical gear wheel compacts having simple cylindrical surface areas, providing each of the cylindrical bodies obtained in this way with a single helical toothing by original molding, namely rolling, and connecting the single helical toothings to each other non-positively by means of press fitting. The manufacturing method includes the steps of original molding, forming and joining. Individual means are required for each of these methods, namely a pressing tool, a sintering furnace, a rolling tool and a joining means. Cost-effective mass production is scarcely possible using such a method.
U.S. Pat. No. 5,865,239 discloses manufacturing gear wheels with a double helical toothing by means of injection molding. The method is described primarily for plastic gear wheels. Reference is, however, made to the fact that it is also intended to be usable for metallic gear wheels. In this case, however, the method would be restricted to the use of casting materials.
U.S. Pat. No. 5,259,744 describes a pressing tool and a pressing method for manufacturing a gear wheel with two helical toothings. The helical toothings can be inner toothings or outer toothings. However, only helical toothings which point in the same direction and exhibit identical helical angles can be obtained using the method and the two devices of this citation.
U.S. Pat. No. 3,694,127 discloses pressing, in one part, a gear wheel with two helical toothings. The pressing tool includes a rotationally mounted matrix comprising a helical toothing on its inner surface area. An upper die and a lower die each comprise an outer toothing, via which they engage with the toothing of the matrix. The hollow cylindrical lower die comprises a further helical toothing on its inner surface area. The two dies are axially moved towards each other via their engagement with the helical toothing of the matrix, in order to compact the powder material. The helical toothing on the inner surface area of the matrix and the helical toothing on the inner surface area of the lower die mould the two helical toothings of the gear wheel. The devices and methods described in this specification, however, are restricted to the manufacture of helical toothings which exhibit different reference circles.
A method and a device for pressing, as one part, a gear wheel with two toothings are known from U.S. Pat. No. 5,698,149. The toothings, each of them single, exhibit an offset with respect to each other by being offset along the circumference of the gear wheel with respect to each other. A pressing tool comprising a lower matrix, an upper matrix, a lower die assembly and an upper die assembly is used for pressing. The hollow cylindrical matrices, movable relative to each other, are provided with toothings on their inner surface areas, on which the toothings of the gear wheel are molded by pressing. For pressing, a compactable material is filled into a chamber formed by the lower matrix and the lower die assembly. Once the chamber has been closed by driving the matrices against each other, the chamber is axially shifted relative to the matrices. The matrices are then rotated about the common longitudinal axis relative to each other, in order to obtain the offset. In the position then assumed by the matrices, the dies of the two die assemblies are moved towards each other and the material is thus compacted. Lastly, the double gear wheel pressing obtained in this way is removed from the tool.
Molding a gear wheel with a multiple helical toothing would be problematic using the device of U.S. Pat. No. 5,698,149. For directly after pressing, when the matrix is necessarily opened, an elastic expansion of the matrix halves would act on the molded part as the pressing force was relieved, said elastic expansion driving the matrix halves axially away from each other. If the matrix halves were fitted with inner surface areas having a helical toothing, these axial forces in the cross-sectional plane, separating the two matrix halves, would result in fractures in the structure of the gear wheel molding. This problem would increase with the size of the gear wheel molding, both with increasing diameter and with increasing axial width.